Changes

config/sys.config: print prefix before msg, if present

It's sometimes desirable to add some context to logging messages,
like the FSM name and identifier, for instance.  With this patch,
this can be easily achieved by passing an arbitrary prefix string
to a logging function/macro:

  %% EXAMPLE: adding metadata to a specific log macro
  ?LOG_INFO("The planet is fine...", #{prefix = "EARTH"}).

or by setting it for all logging of a process:

  %% EXAMPLE: adding context for the whole process
  logger:set_process_metadata(#{prefix => "E-RAB FSM"}).

Change-Id: Ib88801b8d38976b98a02ed06e1638e0e9056bafb